Add Travis CI shield.io badge

[IAlibay]

Now that #3149 has been enabled with a weekly Travis CI cron job for aarch64 and ppc64le, I thought it might be worth re-adding a Travis CI status badge on our CI.

To make it more obvious as to what that badge relates to, I used shields.io to create a custom labelled shield.

Changes made in this Pull Request:

• Adds shield.io version of Travis CI badge to README.rst

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

[codecov]

Codecov Report

Merging #3158 (6ba4eb8) into develop (3f589e3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #3158   +/-   ##
========================================
  Coverage    92.68%   92.68%           
========================================
  Files          168      168           
  Lines        22677    22677           
  Branches      3212     3212           
========================================
  Hits         21018    21018           
  Misses        1611     1611           
  Partials        48       48           

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[lilyminium]

Love the green! Thank you @IAlibay